1. Field of the Invention
The present invention relates to a heat-sensitive stencil sheet. Specifically, it relates to a heat-sensitive stencil sheet which is preferable to stencil-making by perforating the same using a thermal head.
2. Description of the Prior Art
A heat-sensitive stencil sheet is prepared by superimposing a thermoplastic resin film on a porous substrate and adhering to each other with an adhesive. A releasing layer for releasing the thermoplastic film from a thermal head is usually provided on the thermoplastic resin film in order to prevent the lowering of the perforating property on account that the thermoplastic resin film is sticked to the thermal head by heating.
In a prior art, a silicone oil having a releasing property (Japanese patent application laid-open No.58-92595), a cold curing type silicone (Japanese patent application laid-open No.59-218893), a thermosetting type silicone (Japanese patent application laid-open No.61-40196), a ultraviolet light curing type silicone (Japanese patent application laid-open No.62-170392) and others have been used as a releasing layer. The silicone oil and the cold curing type silicone have, however, the problems that these silicone oils are transferred to the porous substrate of the other heat-sensitive stencil sheet, when they are stacked with each other. The thermosetting type silicone also has the problems that the silicone is readily transferrable to the laminated porous substrate, wrinkle appears on the surface of the thermoplastic resin film by heating and the heat-sensitive stencil sheet is curled. The ultraviolet light curing type silicone has the disadvantage that a curing failure can be easily occurred when reaction inhibiting materials are intermingled therein.
Some methods have been known that an antistatic agent (Japanese patent application laid-open No.2-9689) and a surface active agent (Japanese patent application laid-open No.60-109888) are coated on the surface of the thermoplastic resin film. These methods are effective to the electrostatic prevention, but they do not give their satisfactory effect to the releasing property of the thermoplastic resin film from the thermal head, resulting in producing nonuniformity in coating them on the thermoplastic resin film.
Furthermore, as a releasing layer, some materials having a melting property, such as higher fatty acid metal salts (Japanese patent application laid-open No.60-19592) and higher fatty acid esters (Japanese patent application laid-open No.63-69695), are also known to be used, but there are the problems that the releasing property is still unsatisfactiory and the molten materials of the thermoplastic resin film are adhered to the thermal head as a residue, resulting in generating stencil-making failures.